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EAA 502EAA 502GIS Data ModelGIS Data Model

Dr. Mohd Sanusi S. Ahamad

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Recall what is a GIS?Recall what is a GIS?• GIS – Geographical Information Systems

• As a concept they are similar to an overhead projector, with a series of transparencies laid upon it.

Overhead projector analogy

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The Data ModelThe Data Model

• Geographical variation in the real world is infinitely complex. Therefore, we require a set of rules (‘the data model’) to convert real geographical variation into discrete objects.

• ‘A set of guidelines for the representation of the logical organisation of the data in a database … (consisting) of named logical units of data and the relationships between them.’

- Tsichritzis, T.C. and Lochovsky, F.H. (1977) Data Base Management Systems, Academic Press: New York.

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Types of data model

The Raster Model

Equivalent of a continuous grid covering the surface, whereby each cell in the grid represents a square on the ground.

The Vector Model

Attempts to represent objects as exactly and precisely as possible by storing points, lines (arcs) and polygons (areas) in a continuous co-ordinate space

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Vector and Raster Models in GIS

Representation of Lines

Raster

Vector

Courtesy of GIS LAB

Vector Data Structure

……Vector data is ideally suited for Computer Mapping and Spatial Database ManagementVector data is ideally suited for Computer Mapping and Spatial Database Management

Raster Data Structure

……Raster data is ideally suited for GIS ModelingRaster data is ideally suited for GIS Modeling

Vector Data Structure

Vector Data Storage Format Series of linked files establishing explicit spatial topology

(relationships among map features)

#F4

Raster Data Structure

Raster Data Storage FormatSingle “flat” file or database table with each data “field” (column)

stored as a separate map (implicit spatial topology)

GIS Database Organization

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TOPOLOGY (for vector data)

• What is topology?• Why is important?• Three types of topological models in GIS• Spatial operations of topology

– Contiguity– Connectivity

• Trade-offs of topological structure• Application model

– Triangular Irregular Network (TIN):Vector-based GIS

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Spatial features and spatial relationships

• Spatial features in maps – Points, lines and polygons

• Human being interprets additional information from maps about the spatial relationships between features– A route trace from an airport to a house– Land contiguity adjacent to streets along which

the lands are located

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The definition of Topology

• The spatial relationships can be interpreted– identification of connecting lines along a path– definition of the areas enclosed within these lines– identification of contiguous areas

• In digital maps, these relationships are depicted using ‘Topology’

• Topology = A mathematical procedure for explicitly defining spatial relationship

• Topology is the description of how the spatial objects are related with spatial meaning

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Topological data models

• Three types of topological concepts– Arc, Node and polygon topologies

• Arc– Arcs have directions and left and right polygons

(=contiguity)

• Node– Nodes link arcs with start and end nodes (=connectivity)

• Polygon– Arcs that connect to surround an area define a polygon

(=area definition)

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Terms and concepts

ToNode

LeftPolygon

RightPolygon

FromNode

Connectivity - from and to nodesContiguity - Polygon EnclosureAdjacency - from Direction

Arc

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Spatial operations of topology

• Connectivity and contiguity (Aronoff, 1989)– A basic, but core spatial analysis operations in GIS

• Contiguity– A biologist might be interested in the habitats that occur next to each other– A city planner might be interested in zoning conflicts such as industrial

zones bordering recreation areas

• Connectivity– Transportation network, telecommunication systems, river systems– To find optimum routings or most efficient delivery routes or the fastest

travel route– To predict loading at critical points in a river channel– To estimate water flow at a bridge crossing that will result from heavy

flood

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Trade-offs of topology

• Advantages– Spatial data is stored more efficiently– Analysis process faster and efficient for large data sets– By topological relationships, we can perform spatial

analysis functions,– Modelling flow through the connection of lines in a

network (i.e. buffering)– Combining adjacent polygons with similar

characteristics (i.e. spatial merge)– Overlaying geographical features (i.e. spatial overlay)

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Disadvantages

• Extra cost and time– creating topological structure does impose a cost– Topology should be always updated when a new map or

existing map is updated

• Additional batch job working– To avoid the extra efforts, GIS systems need to run a

batch job (i.e. a process that can be run without user interactions); 70% of total GIS costs

– Autoexec.bat in DOS– Macro languages such as AML (Arc/Info), Avenue

(ArcView), MapBasic (MapInfo) and etc

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Conclusions of topology

• When topology is created, we can identify

– Know its positions of spatial features

– Know what is around it

– Understand its geographical characteristics by virtue of recognising its surroundings

– Know how to get from A to B

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MetadataMetadata

• Metadata is ‘data about data’. It should include such information as:-– The origin of the data source– When the dataset was created– Who created / modified it– Data coverage and scale– Accuracy and precision– Ownership, copyright and restrictions on use– Dataset file location

• Critical to maintaining an effective GIS

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Relevant ReadingRelevant Reading• Aronoff, S. (1989) Geographic information systems :a

management perspective, WDL Publications: Ottawa. • Burrough, P. A. (1986) Principles of geographical

information systems for land resources assessment, Clarendon Press: Oxford.

• Burrough, P.A. and McDonnell, R.A. (1998) Principles of geographical information systems, Oxford University Press: Oxford.

• Heywood, I. Cornelius, S. and Carver, S. (1998) An Introduction to Geographical Information Systems, Longman: Harlow.

• Longley, P.A. et al (1999) Geographical Information Systems. Second edition. Wiley: Chichester.

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Journals

• GEOEurope (formally G.I.S. Europe)

• GI News / Mapping Awareness

• G.I.S. World

• International Journal of GIS

• Photogrammetric Engineering and Remote Sensing

• Transactions in GIS